Add a manual test for providing a stable state in alert/confirm/prompt

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24707
Update the stable state in dialogs test to match spec change

http://html5.org/r/8620
Add a manual test for providing a stable state in print()

Gecko currently fails this test.
Add a manual test for providing a stable state in beforeunload
Merge pull request #970 from foolip/stable-state-alert

Add a manual test for providing a stable state in alert/confirm/prompt